I'd think you'd be better off just forgetting about the extra 1 meg resistor. If you add it in on one end of the pot you won't be able to turn the volume down low enough, and on the other end you won't be able to turn it high enough for those faint shortwave signals.
Although using a 1 meg pot instead of 2 meg pot would have some theoretical significance, I think anyone would be really hard pressed to notice any difference from a practical standpoint. Connecting the 51K resistor to the center tap would be, as Norm says, your best bet if you can't find a pot with the extra tap.
Alternatively, Mark Oppat at www.oldradioparts.net
is often able to come up with the correct pot with the extra tone compenpsating tap. He has a large store of such controls.
According to philcoradios.com the 37-630 was quite similar to the 37-620 with the exception that yours has a shadow meter.